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'[EE]: Measuring AC mains with a O-scope.'
2003\02\16@062127 by erholm (QAC)

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Hi.
I'v looked in the O-scope docs and on the net, but
havn't come up with an answer, so I'm trying here...

Have just bought a HP 54602B O-scope.

Now, is it "safe" to connect a probe directly to the live
wires of the mains ? The max voltage on the scope is 600V, but
I'm thinking more about a (possible) connection between the
probe ground and the ground connection on the mains cable of
the scope.

I'v seen on the net that e.g. Tektronics has some "isolation"
probes (that doesn't look cheap, b.t.w). Whould that be needed ?

Possible a real O-scope beginners questions, but then, that's
what I am...

Regards
Jan-Erik Soderholm

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2003\02\16@065941 by Russell McMahon

face
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> Have just bought a HP 54602B O-scope.
> Now, is it "safe" to connect a probe directly to the live
> wires of the mains ? The max voltage on the scope is 600V, but
> I'm thinking more about a (possible) connection between the
> probe ground and the ground connection on the mains cable of
> the scope.

WIRE yes.
WIRES maybe not.
ie IF mains ground is at scope ground (connected physically) then grounding
the probe ground and connecting the probe tip to mains is PROBABLY OK. This
will vary with probe. It may be wise to check probe voltage rating but I
suspect that all will be mains rated.

If your mains is not ground referenced (as happens (unwisely) in a few
areas), you may be tempted to connect probe tip to one wire and scope ground
to other mains wire. YMMV BUT this is not a good recipe for scope or
operator survival. If you understand exactly what connects where, and what
voltage is tolerable where, this may be OK but if you are not, steer clear.

If the scope has a differential mode you can use two probes and connect one
probe tip to each mains wire PROVIDED the scope is rated to a high enough
differential voltage level.




           Russell McMahon

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2003\02\16@072942 by

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With "mains ground", do you mean the third ground connection ?
I think that in the US, they have plugs that make a differense
between the two "live" wires, but here we can turn the plugs
anyway round. So we have two wires with 220v and one protective
ground connection.

Anyway, the manual for the scope says "Maximum input voltage:
400V (dc + peak ac)"

And the data sheet for the probe says "Max input (dc + peak ac)
450V". (It's two 10:1 probes)

The scope has "ch1 + ch2" and "ch1 - ch2" modes.

I'm experimenting with a "transformer-less" low power 5v supply
with a resistor a X2 cap a zener and a few smaller lowvoltage caps.
The "0v" wire of the 5v supply, is the same as one of the mains
wires.

Jan-Erik.

{Original Message removed}

2003\02\16@085826 by Patrick J

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From: "Jan-erik Svderholm
>So we have two wires with 220v and one protective ground connection.

Ehrm, no we don't. What do we have in Sweden is:
1 wire with 240VAC
1 wire that is connected to ground (refered to as 'nollan')
1 wire that is protective ground (green-yellow wire)

Protective ground is actually connected to the same point in the fuse-box,
but separeated throughtout the house.

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2003\02\16@090817 by

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Yes, but in practice, you can turn the plug anyway round,
so the 220/240VAC wire and the "nollan" could be any one
of the two. You never actualy know.

Does this change the way one could connect O-scope probes ?

Jan-Erik.

Patrick J wrote:

{Quote hidden}

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2003\02\16@095319 by John Ferrell

face picon face
A direct connection is really not neccesary. The safest way would be to
measure with resistor(50K) in series with the probe. The high impedance of
the scope will make the resistor invisible.
First protect yourself, second, protect your scope!

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2003\02\16@195954 by Patrick J

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From: "Jan-erik Svderholm (QAC)"
> Yes, but in practice, you can turn the plug anyway round,
> so the 220/240VAC wire and the "nollan" could be any one
> of the two. You never actualy know.
True, you can do that.

> Does this change the way one could connect O-scope probes ?
If your scope has internal grounding... it would blown to hell.

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2003\02\16@214755 by Jinx

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> > Does this change the way one could connect O-scope probes ?

> If your scope has internal grounding... it would be blown to hell

I wouldn't even think of measuring mains without an isolating
transformer. I have a small 150VA one that goes on either the
scope or the circuit

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2003\02\17@024451 by

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I think I'll go for an isolation transformer, and/or
the 50k/100k resistors in series with the probes
mentioned in another post.

Thanks all !
Jan-Erik.

{Original Message removed}

2003\02\17@034139 by Anand Dhuru

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Here's a page that addreses your issue, gives a neat solution too:

http://www.geocities.com/ido_bartana/oscope.htm


Regards,

Anand Dhuru

----- Original Message -----
From: "Jan-erik Söderholm (QAC)" <spam_OUTJan-erik.SoderholmTakeThisOuTspamPAC.ERICSSON.SE>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Monday, February 17, 2003 01:14 PM
Subject: Re: [EE]: Measuring AC mains with a O-scope.


> I think I'll go for an isolation transformer, and/or
> the 50k/100k resistors in series with the probes
> mentioned in another post.
>
> Thanks all !
> Jan-Erik.
>
> {Original Message removed}

2003\02\17@035231 by

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Thanks !
Nice link.
But, in this case, you're expected to *know* which
wire is "line" and which is "neutral". Here in Sweden, as
in most parts of Europe, we can turn the plug anyway round,
so there is no way of knowning what's "line" and whats "neutral".

Jan-Erik.

-----Original Message-----
From: Anand Dhuru [ardhuruspamKILLspamVSNL.COM]
Sent: den 17 februari 2003 09:38
To: .....PICLISTKILLspamspam.....MITVMA.MIT.EDU
Subject: Re: [EE]: Measuring AC mains with a O-scope.


Here's a page that addreses your issue, gives a neat solution too:

http://www.geocities.com/ido_bartana/oscope.htm


Regards,

Anand Dhuru

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2003\02\17@104204 by Alan B. Pearce

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>Here's a page that addreses your issue, gives a neat solution too:

>http://www.geocities.com/ido_bartana/oscope.htm


Gee the whole PICList must have hit it - I get a "Site has exceeded
allocated data transfer" type of error on Monday morning. :))) Guess the
site owner will have to stump up some more money.

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2003\02\17@104213 by Spehro Pefhany

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At 09:50 AM 2/17/2003 +0100, you wrote:
>Thanks !
>Nice link.
>But, in this case, you're expected to *know* which
>wire is "line" and which is "neutral". Here in Sweden, as
>in most parts of Europe, we can turn the plug anyway round,
>so there is no way of knowning what's "line" and whats "neutral".

If it blows the ground wire off your probe, turn the plug around. ;-)

More seriously, a multimeter from wire to earth ground will tell you before
the fireworks start. Many of us have, at one time or another, run a scope
"hot" out of necessity (using naughty methods such as breaking the ground
wire), but it is NOT recommended and could easily kill you if you don't know
just what you are doing or if you make a mistake out of being tired etc.

I'd never make such a mistake, because I visualize current and potential
like a general visualizes troop movements, but not everyone does that all
the time.

OT: http://www.idleworm.com/nws/2002/11/iraq2.shtml (from CNN)

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2003\02\17@114559 by llile

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No Isolation Trasformer = BOOM! or Yeeoooooow!

Isolation transformer = OK

Hook the isolation transformer to your device under test, not your scope.
If you hook the isolation transformer to your scope, the scope ground will
become live when you hook it to your circuit.  I shocked the livin'
%$&#%&$% out of my boss this way once, he leaned on the scope chassis and
touched the meter frame of my bench.  Not good for one's long-term
employment prospects.

Another time I hooked a scope to a TV with one of the mains wires
connected directly to the chassis.  Two wire cords should be outlawed, but
the US lags behind everywhere else in this regard.   BOOM! so much for my
scope ground lead. Also not good for one's long term employment prospects.


Expensive differetnial probes are unneccesary IMHO.  I have one I've never
used.

--Lawrence






Jinx <joecolquittspamspam_OUTCLEAR.NET.NZ>
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02/16/2003 08:45 PM
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       cc:
       Subject:        Re: [EE]: Measuring AC mains with a O-scope.


> > Does this change the way one could connect O-scope probes ?

> If your scope has internal grounding... it would be blown to hell

I wouldn't even think of measuring mains without an isolating
transformer. I have a small 150VA one that goes on either the
scope or the circuit

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2003\02\17@123213 by Ray Gallant

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----- Original Message -----
From: <RemoveMEllileTakeThisOuTspamSALTONUSA.COM>
To: <spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU>
Sent: Monday, February 17, 2003 12:45 PM
Subject: Re: [EE]: Measuring AC mains with a O-scope.


{Quote hidden}

On scopes that have differential input mode between two channels, what is
the max voltage that can be monitored?  The Vmax of one channel or both
combined (Vmax x 2)?  {slewrate}

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2003\02\17@160330 by Jinx

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> Hook the isolation transformer to your device under test, not
> your scope. If you hook the isolation transformer to your scope,
> the scope ground will become live when you hook it to your circuit.

That's quite true, better to use the IT on the circuit . The one I
have is only 150VA which rules out using it on anything that takes
more than that (eg if the load is a motor / heater / lighting), but it's
enough to run the scope if I have to do it the dodgy way around.
Nothing quite like having a "live" ground for focussing the mind
or tightening the sphincter

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2003\02\17@164244 by Chris Loiacono

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> Nothing quite like having a "live" ground for focussing the mind
> or tightening the sphincter

Maybe that's my problem.....

Chris
(5x survivor of 460V+ shorts, N floats and/or ground faults....)

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2003\02\17@231707 by Roman Black

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Chris Loiacono wrote:
>
> > Nothing quite like having a "live" ground for focussing the mind
> > or tightening the sphincter
>
>  Maybe that's my problem.....
> Chris
> (5x survivor of 460V+ shorts, N floats and/or ground faults....)


What?? That your mind is too focused? Or...?
;o)
-Roman

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'[EE]: Measuring AC mains with a O-scope.'
2003\03\17@042940 by
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Now, lets take a second round on this issue...

I think it's clear that I need a isolation transformer.
It also looks like it would be best to connect the IT
between the mains and the device under test, not to the
scope, right ?

I have checked a few electronics catalogs, and the IT
seems to be from $150 USD and up starting with aprox 300VA
units.

Now, my device under test is realy low power. It's a
"transformerless power supply" with a "secondary" voltage
of 5V. Maybe 5-10 mA in average on the 5V rail. Just a 400V
"X2" cap, a few resistors, a low voltage smothing-cap, a zener
and a 78L05. I does work, a get 5V as messured with a battery
powered voltmeter, but I'd like to "see" what what the 5V
rail "looks like" under load.

So, would it be possible to put together something
inexpensive either with some ready made (smaller)
transformar, or buy handwinding something ?
Any design tips ?

Jan-Erik.



llileEraseMEspam.....SALTONUSA.COM wrote:
>No Isolation Trasformer = BOOM! or Yeeoooooow!
>Isolation transformer = OK
>Hook the isolation transformer to your device under test, not your scope.
>If you hook the isolation transformer to your scope, the scope ground will
>become live when you hook it to your circuit.

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2003\03\17@055045 by cdb

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On Mon, 17 Mar 2003 10:28:16 +0100, Jan-erik Söderholm (QAC) wrote:
So, would it be possible to put together something
inexpensive either with some ready made (smaller)
transformar, or buy handwinding something ?
Any design tips ?

Why not try two microwave transformers back to back out of a defunct oven.

I have a surplus Toroidal 230-230 transformer for this sort of thing - it only allows about 150mA though.

Colin
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2003\03\17@070917 by Ray Gallant

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I would like to add to the question on this topic which was not answered
before.  While measuring AC mains with a two channel scope in differential
mode (just probes tips, no probe grounds), what is the maximum voltage that
can be measure if the spec for a single channel is 400Vpeak?


{Original Message removed}

2003\03\17@072943 by George Smith

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On 17 Mar 2003, at 8:07, Ray Gallant wrote:

> I would like to add to the question on this topic which was not answered
> before.  While measuring AC mains with a two channel scope in differential
> mode (just probes tips, no probe grounds), what is the maximum voltage that
> can be measure if the spec for a single channel is 400Vpeak?

Assuming UK mains, one side is at or near earth potential.
With your scope earthed, one probe will still see full mains voltage across it
and the other will see just the difference between your local earth and the
earth at the substation.
If the supply you were measuring were truly isolated then each probe could
retain it's 400v rating (to scope body=earth) giving a total of 800v.


George Smith

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2003\03\17@073603 by George Smith

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On 17 Mar 2003, at 10:28, QAC wrote:

> I think it's clear that I need a isolation transformer.
> It also looks like it would be best to connect the IT
> between the mains and the device under test, not to the
> scope, right ?
Absolutely.


> Now, my device under test is realy low power.
<snip>
> So, would it be possible to put together something
> inexpensive either with some ready made (smaller)
> transformar, or buy handwinding something ?
> Any design tips ?
Used to be that we could do this for low power with a couple of 6.3v heater
transformers wired back to back. But you could do the same for your light
load with a pair of any transformers. (Pull a couple of cheap wall warts apart).


George Smith

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2003\03\17@091236 by John Ferrell

face picon face
I have had a pair of transformers in back to back service for over 20 years
with no problems.

Before IBM offerred a color monitor I had a TI-99 color computer. I
purchased a Sony portable TV as a monitor. It lacked isolation and the back
to back transformers offerred the cheapest solution. It remains in service
as a TV in my workshop.
John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2003\03\17@094835 by Lawrence Lile

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OK, I have done it this way.  One side of your device under test is tied to Neutral (earth) and your scope ground is (supposedly) tied to the same (supposedly) neutral.   Carefully take a meter and measure the potential between these two grounds.  If there is less than a volt or two, they are OK to hook together.  But add a 250ma fuse in there anyway to protect your
scope ground lead in case you made a mistake.  Also, make sure that the ground connection also measures low voltage to a handy water pipe or something else that is earthed.  Electricians randomly switch the neutral and hot wires on electrical plugs just to keep us guessing.
-- Lawrence Lile





Jan-erik Söderholm (QAC) <RemoveMEJan-erik.SoderholmKILLspamspamPAC.ERICSSON.SE>
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        To:     spamBeGonePICLISTSTOPspamspamEraseMEMITVMA.MIT.EDU
       cc:         Subject:        Re: [EE]: Measuring AC mains with a O-scope.


Now, lets take a second round on this issue...

I think it's clear that I need a isolation transformer.
It also looks like it would be best to connect the IT
between the mains and the device under test, not to the
scope, right ?

I have checked a few electronics catalogs, and the IT
seems to be from $150 USD and up starting with aprox 300VA
units.

Now, my device under test is realy low power. It's a
"transformerless power supply" with a "secondary" voltage
of 5V. Maybe 5-10 mA in average on the 5V rail. Just a 400V
"X2" cap, a few resistors, a low voltage smothing-cap, a zener
and a 78L05. I does work, a get 5V as messured with a battery
powered voltmeter, but I'd like to "see" what what the 5V
rail "looks like" under load.

So, would it be possible to put together something
inexpensive either with some ready made (smaller)
transformar, or buy handwinding something ?
Any design tips ?

Jan-Erik.



KILLspamllilespamBeGonespamSALTONUSA.COM wrote:
>No Isolation Trasformer = BOOM! or Yeeoooooow!
>Isolation transformer = OK
>Hook the isolation transformer to your device under test, not your scope.
>If you hook the isolation transformer to your scope, the scope ground will
>become live when you hook it to your circuit.

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2003\03\17@133749 by

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Well, you can plug in *our* plugs either way round, so
it doesn't matter how the wires are connected in the wall
(or in the plug)

I could measure what's what, but the next time I might turn
the plug the other way round, and...

The isolation transformer way sound a bit safer, I think.

I think I'll try the sugestion about using a couple of small
transformers from two wall warts...

Thanks all!
Jan-Erik.



Lawrence Lile wrote :
> Electricians randomly switch the neutral
> and hot wires on electrical plugs just to keep us guessing.

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2003\03\17@140036 by Patrick J

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om du har ett osilloscope som inte har jordfvrbindelse internt
se kan du mdta direkt har jag fvr mig.


From: "Jan-erik Svderholm
> The isolation transformer way sound a bit safer, I think.
> I think I'll try the sugestion about using a couple of small
> transformers from two wall warts...

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2003\03\17@210816 by Dwayne Reid

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At 10:28 AM 3/17/03 +0100, QAC wrote:
>Now, lets take a second round on this issue...
>
>I think it's clear that I need a isolation transformer.
>It also looks like it would be best to connect the IT
>between the mains and the device under test, not to the
>scope, right ?

Yes to both.

>So, would it be possible to put together something
>inexpensive either with some ready made (smaller)
>transformar, or buy handwinding something ?

As others have mentioned, back-to-back transformers work reasonably well.

You want to use reasonably large transformers - something in the range of
120 - 200 VA for best results.  Using transformers that are too small
results in a surprisingly large source impedance.  This can really skew
your test results if you are doing anything even remotely similar to a
switch mode type power supply.

I'm assuming that you have 230 Vac mains.  Try to pick transformers with
the largest secondary voltage you can find - 24 Vac and up should be OK.

dwayne

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